One of the latest developments in PFC topologies is the totem pole PFC. The active, bridgeless topology replaces the diode-rectifying bridge of the previously shown passive topology with switches. These additional power switches are typically silicon (Si), silicon carbide (SiC), or gallium nitride (GaN). Here, the focus is on Si and SiC FETs for this topology. Silicon carbide (SiC) FETs only have two junctions in series (as opposed to three in the boost topology). Utilizing SiC FETs allows for faster switching and lower reverse recovery charge which leads to reduced switching losses. Figure 2-4 and Figure 2-5 show two configurations for the totem pole PFC topology.

Figure 2-4 Totem Pole PFC Circuit Utilizing Diodes for Line Rectification

Figure 2-5 Totem Pole PFC Circuit Utilizing MOSFETs for Line Rectification

The totem pole topology has two branches of switching, the right-side branch with either two silicon (Si) MOSFETs (Q3 and Q4 in Figure 2-5) or two diodes (D3 and D4 in Figure 2-4) and the left branch with two SiC FETs (Q1 and Q2 in Figure 2-4 and Figure 2-5). The right branch, called the slow leg, provides line rectification of the AC signal at grid frequency (typically 50 or 60Hz). The slow leg is in a half-bridge configuration and so a 600V half-bridge driver is needed. This voltage rating is due to these circuits typically outputting around 400V DC. The left branch, the fast leg, which switches at high frequencies (100 to 250kHz), is used step up the voltage and shape the input current. At these high switching frequencies, operational difficulties occur with noise-creating transients, adding inefficiencies. These transients are due to the differential voltage between the two separate ground references. The gate driver’s tolerance to this noise is called common-mode transient immunity (CMTI). Isolated gate drivers provide high CMTI, therefore, an isolated dual-channel gate driver is needed to output the high current needed for the SiC power FETs and maintain maximum efficiency.

For the slow leg, a 600V half-bridge gate driver that provides high drive current and fast switching characteristics to drive the high-power switches used is needed. The UCC27714 is a 600V, non-isolated driver with high output current capabilities of 4A sink and 4A source peaks. The low propagation delay of 90ns and delay matching of 20ns as well as the rise and fall times of 15ns on HO and LO helps achieve the maximum efficiency that is required of these systems.

The SiC FETs that are often used on the fast leg require low delay times, high CMTI and high current to maximize efficiency. The UCC21551 is an example of a driver that meets the needs of the fast leg. The UCC21551 has a CMTI rating of 125V/ns and drive current of 4A source and 6A sink. The UCC21551 also has the fast switching characteristics needed such as a 33ns propagation delay and 5ns maximum delay matching. The input side isolation of the UCC21551 is rated at 5kV_RMS peak-reinforced isolated barrier as well as UVLO protection of 12V or 17V.